1. Field of the Invention
The present invention relates to an imaging lens, composed of three sheets of lenses, for reading out images, which is used in an optical system of an image readout apparatus such as facsimile machine or image scanner, for example.
2. Description of the Prior Art
It is basically necessary for an image readout lens used in a facsimile machine or image scanner of a type in which an original image forms a reduced or magnified image on an imaging device such as CCD to have a high resolution at an imaging magnification used, a large amount of corner illumination, and minimized distortion. In addition, in response to the recent demands for reducing the size and cost of the optical apparatus as a whole, the lens is required to minimize its size and manufacturing cost.
Known as an imaging lens which can respond to such requirement are three-sheet lenses disclosed in Japanese Patent Publication No. 2-39765 and Japanese Unexamined Patent Publication No. 2-42411.
In the prior art disclosed in the above-mentioned Japanese Patent Publication No. 2-39765, however, since the lens has a large angle of view, image surface curvature may be so large that it is difficult to form images of three colors of blue, green, and red on the same plane, thus failing to be used satisfactorily for reading out color images.
In the prior art disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 2-42411, on the other hand, though brightness is sufficient, angle of view is rather small.
Further, in the prior art disclosed in the Japanese Unexamined Patent Publication No. 63-307414 relates to a photographic lens in a compact camera or like.
In the case of a photographic lens for a camera, even when the focal length is short, an extremely wide angle is not required.
By contrast, in the case of a readout lens for a facsimile machine, image scanner, or the like, an extremely wide angle is required.